When designing with electronics it is pretty common to select the closest value as you've done. It will probably work fine, but you could always do the math in reverse, and see what the part will do for your application.

When designing with electronics it is pretty common to select the closest value as you've done. It will probably work fine, but you could always do the math in reverse, and see what the part will do for your application.

You calculated the value you needed. Now calculate what the value you've chosen will do. If you are powering up an LED, for example, see what the current with the new value will do. It will be very, very close. This is more an exercise in math, the part will work fine.

You calculated the value you needed. Now calculate what the value you've chosen will do. If you are powering up an LED, for example, see what the current with the new value will do. It will be very, very close. This is more an exercise in math, the part will work fine.

Click to expand...

I guess, I have done math reverse.

Wait, why when I get my multimeter and read the voltage of the battery (I put already the resistor), the value is still 13.06 something close to that. But I am wondering why when I connect it to the radio, the radio runs and it doesn't explode as what I expected because the volts is 13.06.

Also, the resistor is becoming hot as time runs, I guess I read it somewhere. But why?

Wait, why when I get my multimeter and read the voltage of the battery (I put already the resistor), the value is still 13.06 something close to that. But I am wondering why when I connect it to the radio, the radio runs and it doesn't explode as what I expected because the volts is 13.06.

Also, the resistor is becoming hot as time runs, I guess I read it somewhere. But why?

Thank ya very much and I hope for any answers.

Click to expand...

Resistors dissipate the power they consume as heat. If it is getting hot then you might want to increase the power rating of the resistor. Instead of a 1/4W resistor, use a 1/2W resistor.

However, the resistor did not pop. I run my radio by 13 volts (with 75Ω resistor already). I run it for more than 15 minutes, or even 30 minutes. It just became hot, but not that popped.

So, why when I get my multimeter and check the readings, the readings was still 13.somethingV not 3.somethingV .

Pls. help...

Catapult

Click to expand...

ok, lets clear this up. where are you measuring with your multimeter? between A & C? Between A& C you will always measure 13V because you are measuring the entire circuit (all the voltage drops). if your resistor is working properly, you should read 3V between B & C and 10V between A & B.

This setup is not proper. Radios are not LEDs, so you can't use the LED formula for this. If your radio has not burned up yet, then it probably has it's own voltage regulator, or it will burn up if you keep using it like that.

ok, lets clear this up. where are you measuring with your multimeter? between A & C? Between A& C you will always measure 13V because you are measuring the entire circuit (all the voltage drops). if your resistor is working properly, you should read 3V between B & C and 10V between A & B.

This setup is not proper. Radios are not LEDs, so you can't use the LED formula for this. If your radio has not burned up yet, then it probably has it's own voltage regulator, or it will burn up if you keep using it like that.

Maybe yes, maybe no. Definitely bad design. But it doesn't HAVE to fail, as long as it's always drawing enough juice such that the voltage at the radio stays within specs, less than say 4V. Data collected so far suggests it might be OK. (It ain't dead YET! ) The B-to-C voltage measurement would help a lot.

Maybe yes, maybe no. Definitely bad design. But it doesn't HAVE to fail, as long as it's always drawing enough juice such that the voltage at the radio stays within specs, less than say 4V. Data collected so far suggests it might be OK. (It ain't dead YET! ) The B-to-C voltage measurement would help a lot.

Resistors resist circuit current. When they do, they develop a voltage drop across them. Resistors are NOT voltage regulators. Use a zener diode circuit or a voltage regulator as stated above. If you are dead-set on using resistors, try a voltage divider circuit.

And the OP did NOT ask for a voltage regulator, nor best practice. He asked whether something would work, and presented data supporting the answer that yes, it might. Only time will tell. Is it good practice or even recommended? No.